The invention relates to stabilizing of a forest work unit and, in particular, to a method and arrangement for stabilizing at least one frame part of the forest work unit.
Various moving vehicles and machines frequently have to work on terrains which are uneven in different ways, as well as on inclined surfaces, whereby it is important to enable, one way or another, sufficient contact of the elements a vehicle or a machine moves on, such as its wheels and/or tracks, with the ground. It is usually desirable to maintain as good a contact with the ground or other working surface as possible for various reasons, such as the propulsion and tractive power of the vehicle, good roadholding between the surface and the vehicle, as low a surface pressure on the ground as possible, or good stability. Such properties are typically particularly emphasized in cross-country vehicles or machines, such as military vehicles or different forestry machines used for mechanical harvesting. These properties may be improved, e.g. by allowing shafts, axles or, particularly in cases of frame-steered machines, separate frames to rotate relative to one another. Different prior art arrangements relating to the rotation of frames of this type are, in fact, known for allowing the rotation and, on the other hand, for preventing or controlling it, when desired, by means of a hydraulically controlled tooth plate connector, friction brake or lock valves and hydraulic cylinders. One such solution is disclosed in publication WO 03055735.
It is thus known to arrange two separate frames of a forestry machine or another moving working machine together by both what is called a frame steering joint enabling steering of the working machine and a rotator joint enabling rotation of the frames relative to one another substantially in relation to the longitudinal axis of the vehicle. One such solution is known from publication EP1261515, for example.
A typical problem with prior art arrangements is that the rotation of frames relative to one another is prevented and allowed on the basis of status data obtained from the traction transmission. Hence, a typical mode of use is to prevent the rotation of the frames relative to one another when the machine is not being driven and, when the machine is being driven, the rotation of the frames is allowed. However, this restricts significantly an eventual use of a crane and its range of operation during drive, as the frame part comprising the crane is not supported in any way. On the other hand, the underlying assumption is that the boom assembly of the machine is only used when the machine is stationary, which significantly reduces productivity in harvester and forwarder operations, for example. Another typical problem with prior art solutions relates to bringing the vehicle to movement when frame rotation has been prevented using a high support moment. When the locking of the frame rotation is released, the frames seek a new position of balance and, when doing so, the frame or frames may swing even very sharply and strongly. Prior art solutions are also typically based on predetermined lockings or attenuation or prevention of rotation between frame parts, and hence they are not capable of reacting to changing circumstances.
A further problem associated with prior art shape-locked toothings and similar non-stepless frame lock implementations is that the lock often becomes locked to a position slightly different from that actually required by the terrain and the position of the machine's frame, which causes instability manifested by a hovering, for example, of the machine on the base. An attempt has been made to solve this problem by stepless locking solutions, such as brake solutions implemented by hydraulic cylinders and based on friction, to allow the locking to be made to a precisely desired rotation of the frames relative to one another. However, even these solutions are not able to take into account variation in the bearing capacity of the terrain under the machine, for example. In addition, strains caused to the ground surface during the works may lead to the surface pressure not being distributed evenly but the machine remains in an instable position, especially when it is stationary for a longer period of time during continued loading or work.